1. Field of the Invention
The present invention relates to a knock detecting apparatus which is used with an ignition timing control system for an internal combustion engine and has the functions of detecting the occurrence of knocking from a knock signal generated internally or externally of any cylinder of the engine due to the cylinder combustion pressure and adjusting the ignition timing of the engine so as to reduce the knocking to a given knock level.
For use with a conventional internal combustion engine, ignition timing control systems have been proposed in which upon noting that the ignition timing has a close relation to the occurrence of knocking, while detecting the knock condition of the engine, the ignition timing is advanced or retarded in accordance with the knock condition and the engine is operated at a weak or so-called trace knock condition, thereby increasing the power output and improving the fuel economy.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows an example of the known knocking detecting apparatus for detecting the occurrence of knocking, which is used in the above type of ignition timing control system.
More specifically, numeral 1 designates a knock sensor of any type capable of detecting a knock induced high frequency signal, e.g., a sensor which detects the occurrence of knocking from the engine block vibrations, the knocking sounds or the cooling water pressure variations.
Numeral 10 designates a knock detecting apparatus whose output is used to advance or retard the ignition timing.
The output of the knock sensor 1 is amplified to a given voltage level by an amplifier circuit 2.
Generally with respect to cylinder internal pressure, high frequency components appear only in the vicinity of the peak value of the internal pressure of the cylinder during the knocking period. Irrespective of whether the knock sensor is of the resonant or nonresonant type, vibration noise (e.g., valve seating noise, ignition noise or the like) also appears at other values than around the peak value of the cylinder pressure. Also, this vibration noise increases with an increase in the engine speed and the S/N ratio deteriorates. Particularly, the effect of the noise is large during the high speed and load operations of the engine. As a result, the usual practice of detecting the presence of knocking has been such that the output of the amplifier circuit 2 is integrated by an integrator circuit 3 to provide a background noise level and this level is compared with the direct output from the amplifier circuit 2 in a comparator circuit 4 thereby determining the presence of knocking. Numeral 5 designates a discriminating circuit whereby a logic level "1" is generated in the presence of knock and a logic level "0" is generated in the absence of knock.
However, generally the signal level of the output from the knock sensor 1 tends to vary in dependence on the engine conditions, particularly the engine speed, the sensor sensitivity, the engine characteristics, etc., and this tendency is not desirable because of it being a primary cause in deteriorating the knock detecting performance.
Thus, another method has been previously proposed in which the amplifier 2 is replaced by an automatic gain control circuit or AGC circuit 6 as shown in FIG. 2. In other words, it is constructed so that the output of the knock sensor 1 is passed through the AGC circuit 6 and its output average value is maintained substantially constant, thereby making the knock detecting sensitivity stable and highly sensitive.
FIG. 3 shows one example of the known type of AGC circuit 6.
In this circuit, when the output from an operational amplifier 6a exceeds, during the negative cycle, the voltage of a Zener diode D.sub.Z, a diode D is turned on and a voltage drop occurs across a resistor R, thereby applying a bias voltage to the gate G of a field effect transistor or FET 6b. In the low voltage range, the FET 6b operates as a variable resistance device and its resistance value is controlled by the gate voltage. Thus, as the bias voltage is applied to the gate G of the FET 6b, a negative feedback is applied to the output amplitude of the operational amplifier 6a so that the gain is reduced and the output of the operational amplifier 6a tends to assume a constant value. In this way, the AGC circuit shown in FIG. 3 is used.
While the knock detecting apparatus shown in FIG. 2 can compensate for changes in the engine conditions and the sensor sensitivity, it still cannot overcome the following disadvantages.
(1) Since the bias voltage applied to the gate G of the FET 6b is dependent on the voltage itself of the operational amplifier 6a, despite the primary intention of controlling the gain of the operational amplifier 6a so as to maintain the background noise level substantially constant, the gain control is also effected in response to a noise voltage of impulsive noise, e.g., ignition noise or valve seating noise which should not be included in the background noise. PA1 (2) Since the above-mentioned bias voltage has no frequency selectivity, the occurrence of knock allows the knock signal itself to affect the gain control. As a result, the continuation of a high knock level has the effect of reducing the background noise unnecessarily.
As described hereinabove, despite the primary intention of maintaining the background noise substantially constant under all the conditions, the abovementioned types of knock detecting apparatus are disadvantageous in that the background noise is varied depending on the presence of knocking and impulsive noise of various kinds.